1. Field of the Invention
The present invention relates to a drive mechanism for a carriage, and, more particularly, to both a lead-screw-driven drive nut mountable on a carriage for universal floating thereon and the assembly of such a drive nut and the carriage.
2. Description of the Prior Art
Some devices, such a s certain types of printers, have reciprocally lead-screw-driven carriages. In such a printer, the direction of carriage motion is back-and-forth across a record, such as a paper sheet, on which printing takes place by the operation of a print head on the carriage. The paper may be drawn over a rotatable platen by either frictional engagement therewith or by engagement of the paper with an associated sprocket wheel. Rotation of the platen to effect controlled line feed advancement of the paper may be achieved by the line feed mechanism of commonly-assigned, copending application of Ingard B. Hodne, Ser. No. 468,048, filed May 8, 1974 and incorporated by reference hereinto.
The carriage may move along one or more guide rods parallel to the lead screw. The carriage and the print head are coupled to the rotatable lead screw by a threaded member, or drive nut, mounted thereon.
Typically, the lead screw is driven by a reversible stepping motor, the rotational movement of the lead screw being translated by the drive nut into linear reciprocal movement of the carriage and the print head. The optical sensor of commonly-assigned, copending application of R. E. LaSpesa, Ser. No. 468,049 filed May 8, 1974 and incorporated herein by reference may be utilized to synchronize carriage position and printer operation with the rotation of the stepping motor.
Lead-screw-driven carriages are typically employed in high speed printers of the dot matrix type wherein wires are selectively "fired" or driven toward the paper to effect printing. Two illustrative printers of the dot matrix type are disclosed in the commonly assigned copending applications of J. L. DeBoo et al., Ser. No. 468,046, filed May 8, 1974 and of J. A. Bellino et al., Ser. No. 512,264, filed Oct. 4, 1974, both incorporated by reference hereinto.
While lead screw drives are generally simpler, more rugged and cheaper than belt or chain drives, they present a number of problems. These problems and some of the prior art solutions thereof are discussed in the commonly assigned, copending application of A. F. Lindberg, Ser. No. 468,047, filed May 8, 1974 and incorporated herein by reference. See also U.S. Pat. No. 3,656,358. A precis of these problems follow.
Because both the lead screw and the drive nut are threaded, unless stringent tolerances are adhered to in the manufacture thereof either some backlash (i.e., spacing between the interfitted screw and nut threads) must be tolerated, or a tight-fitting drive nut must be employed.
Attempts to manufacture the lead screw and drive nut to stringent tolerances, i.e., less than .+-. 0.0015 inches, have proven to be impractical for a number of reasons, first, the lead screw extends across the entire width of the printer parallel to the platen. Thus, there is always a tendency for the lead screw to develop a slight bow which is most pronounced along the intermediate region thereof. Second, while the lead screw is normally mounted in precision bearings, tolerance variations in the bearing mountings usually lead to misalignment between the lead screw and carriage guide rods. Third, because of the size of the threads and the length of the lead screw, formation of the threads by precision machining, as distinguished from conventional (and not so precise) cold rolling, is prohibitive from a cost standpoint.
Accordingly, even if a drive nut could be manufactured to accurately engage the lead screw with a very close fit, having negligible backlash, very high frictional forces develop not only between the lead screw and drive nut, but also between the carriage and the guide rods. Such frictional forces lead to excessive wear of the mating parts generating them, and may overcome the driving torque of the stepping motor, in which case, the carriage binds on the guide rods. Such a condition, of course, may seriously damage the stepping motor.
Equally important is the fact that any non-uniform frictional forces, whether or not great enough to actually bind the carriage, vary the speed at which the carriage is moved along the guide rods. Such unintended variations in carriage speed during printing cannot be tolerated in high speed dot matrix printers, because there must be a very precisely correlated relationship between the firing of the print wires and the position of the print head.
In an attempt to solve some of the foregoing problems, drive nuts have been proposed wherein the central bore is threaded along its entire axial length, but with one end region thereof formed with a circumferentially spaced array of slits to produce a plurality of internally threaded, cantilevered resilient fingers. One or more so-called garter springs may be mounted on the fingers to augment the compressive forces of the resilient fingers on the threads of the lead screw. See U.S. Pat. No. 3,656,358.
In still another design, a drive nut is formed with an intermediate thin wall section having a circumferential array of longitudinally disposed slits formed therein and an end section that is slightly tapered. This design allows a variable degree of expansion of the drive nut body over an appreciable portion of the axial length thereof.
In these drive nut designs, the central bore is threaded along its entire length, which prevents the drive nut from being slightly tilted or skewed relative to the axis of the lead screw. Such tilt or skew is often desirable to compensate for bow in the lead screw, as well as for any misalignment thereof relative to the carriage guide rods.
Another approach to the problem of minimizing frictional forces between a drive nut and a lead screw has been to purposely build-in a predetermined degree of backlash therebetween. Prior backlash-containing drive nuts are typically elongated, solid wall, tubular members with a threaded bore extending along the entire axial length thereof. Such a construction prevents any appreciable tilting or skewing of the drive nut relative to the axis of the lead screw.
When a built-in degree of backlash is employed in a lead screw-driven drive nut a substantial degree of kinetic energy is established by movement of the mass of the coupled carriage, together with any associated apparatus carried thereby, such as the print head. Such kinetic energy can establish large, initial impact forces, as well as transient forces, between the lead screw and drive nut threads if not compensated for or absorbed in some way. These forces, may lead to "bouncing" of the carriage (and print head) which has proven to be particularly troublesome in lead screw driven printers where the carriage is moved from one character print column position to the next across the width of the platen in step-by-step fashion.
In the above-noted Lindberg application there is disclosed a lead screw-driven drive nut which solves many of the above prior art problems. The drive nut has an axial bore which is threaded for approximately one-half of its length, and which is unthreaded along the remaining half of the bore. An annular clearance space is established between the unthreaded portion wall of the bore and an associated lead screw passing there-through, which space allows the drive nut to acquire a slightly tilted or skewed orientation, relative to the axis of the lead screw. A resilient mounting assembly for the drive nut includes a specially constructed resilient O-ring which is coaxially positioned on the drive nut near the unthreaded end thereof and, in combination with a pair of adjustable O-ring clamping plates, resiliently mounts the drive nut in a cantilevered manner on an apertured side wall of the carriage through which the lead screw passes. The drive nut may be slightly skewed or tilted relative to the axis of the lead screw, while still minimizing any relative axial and/or radial displacement therebetween. With the drive nut threads additionally dimensioned so as to establish a predetermined degree of backlash when mounted on the lead screw, the drive nut-carriage assembly readily compensates for both tolerance variations in the lead screw threads, and any bow therein, as well as any lack of parallelism between the lead screw and the guide rods of the carriage. The O-ring may be made of a viscoelastic material, such as a polyester base urethane, so that any kinetic energy-imparted bounce forces that are established by the carriage due to backlash (whether predetermined or otherwise) are substantially, if not completely, absorbed by the O-ring, and dissipated thereby in the form of heat.
The drive mechanism of the present invention is a simplified alternative to, and an improvement of, the drive nut of the above-noted Lindberg application. It avoids the problems of the prior art, as will be subsequently appreciated, and is simpler and cheaper to make, install and maintain, than the Lindberg drive nut.
It is therefore an object of the present invention to provide a new and improved drive mechanism for a carriage.
Yet another object of the present invention is to provide a new and improved drive mechanism including a lead-screw-driven drive nut on a carriage for universal floating thereon.
Another object of the present invention is the provision of a drive mechanism including a drive nut which is mounted to both a lead screw and to a movable carriage for reciprocating the carriage upon rotation of the lead screw in a manner such that bow in the lead screw and movement of the carriage other than parallel to the lead screw are compensated for to minimize excessive wear of the lead screw and the drive nut.
Another object of the present invention is the provision of a new and improved drive mechanism including a drive nut for a lead-screw-driven carriage wherein the nut is free to float in a direction at right angles to the direction of movement of the carriage and to the axis of the lead screw to compensate for alignment errors and to exert forces on the carriage due to rotation of lead screw only substantially parallel to both the axis of the lead screw and the direction of movement of the carriage.
Another object of the present invention is the provision of a new and improved drive mechanism including a drive nut for a lead-screw-driven carriage which obviates the difficulties of the prior art.